The present invention relates to an optical signal repeater at a wavelength division multiplex (WDM) system, in particular, which can control an output level of each optical signal of WDM optical signals to be outputted to a designated value, and an output level control method used this repeater.
Japanese Patent Application Laid-Open No. HEI 9-326770 discloses a linear repeater using for a WDM system. In this application, at an output stage of an optical amplifier in which WDM optical signals to be transmitted are amplified, a part of the amplified optical signals is split at an optical splitter. These WDM optical signals split at the optical splitter are inputted to a variable wavelength optical filter that can variably control transmitting wavelength to be transmitted in each optical signal. An optical signal transmitted through this variable wavelength optical filter is inputted to an automatic gain control (AGC) section, and the gain of each optical signal is controlled to a designated value by monitoring the power level of the optical signal inputted to the AGC section.
FIG. 1 is a block diagram showing a conventional optical signal repeater using for a WDM system. This conventional optical signal repeater consists of an optical fiber transmission line 11 in which WDM optical signals are transmitted, an optical amplifier 12 which amplifies the transmitted WDM optical signals, an optical multiplexer 13, an optical splitter 14 which splits the amplified WDM optical signals outputted from the optical amplifier 12 into two parts, a pumping laser diode (LD) 17 being a pumping light source, a variable wavelength optical filter 18 which can variably control transmitting wavelength of the WDM optical signals to be transmitted in each optical signal, and an AGC section 19. FIG. 2 is a block diagram showing gain control operation of the conventional optical signal repeater shown in FIG. 1. In FIG. 2, a broken line from the pumping LD 17 shows a flow of pumping light and the other broken lines show flows of optical signals. And spectra of the optical signals are also shown in FIG. 2. In these spectra, the vertical axis shows the optical power level and the horizontal axis shows the optical wavelength.
At this conventional optical signal repeater, the output level of each optical signal of the transmitted WDM optical signals is controlled to a designated level by using the following method. That is, a part of the WDM optical signals outputted from the optical amplifier 12 is split at the optical splitter 14, and each optical signal of the split WDM optical signals is transmitted through the variable wavelength optical filter 18. The transmitted optical signal is inputted to the AGC section 19, and the AGC section 19 controls the output from the pumping LD 17 so that the power level of each optical signal inputted to the AGC section 19 is kept in a designated level.
At this kind of optical signal output level control method, the AGC section 19 controls the output of the pumping LD 17 so that the output level of each optical signal of the WDM optical signals becomes a designated level, without any relation with the power level of the WDM optical signals inputted to the optical signal repeater. Therefore, each optical signal transmitting through the optical fiber transmission line 11 is amplified to a designated level without any relation with the power levels of the inputted WDM optical signals at the optical fiber transmission line 11. Consequently, it is possible to realize a WDM optical signal transmission system in which the fluctuation of the outputting optical power level of each optical signal is reduced.
However, at the conventional optical signal repeater mentioned above, there are several problems. First, at the conventional optical signal repeater, a part of the optical signals is transmitted through the variable wavelength optical filter 18 and inputted to the AGC section 19. And the variable wavelength optical filter 18 provides a driving unit (not shown) which varies the transmitting wavelength, however, there is a problem that this driving unit does not have high reliability.
Secondly, a circuit, which controls the transmitting wavelength through the variable wavelength optical filter 18, is additionally required, as a result, the structure of the optical signal repeater becomes complex.
Thirdly, at the variable wavelength optical filter 18, it is difficult to make the transmitting wavelength band narrow, therefore optical signals adjacent to the optical signal to be transmitted leak and the leaked adjacent optical signals are inputted to the AGC section 19. As a result, it is difficult to monitor the input level of each optical signal to the AGC section 19 precisely.
Moreover, in order to avoid making the adjacent optical signals input to the AGC section 19, it is required that the adjacent wavelengths are positioned at the far positions from the optical signal to be transmitted. Therefore, there is a problem that the number of the wavelengths to be multiplexed is limited in the amplifying wavelength band.
It is therefore an object of the present invention to provide an optical signal repeater and an output level control method used this repeater, in which the output level of each optical signal of the WDM optical signals does not change even when the number of inputted WDM optical signals changes, and which is able to have high reliability and can be a simple structure, further is applicable for a system in which intervals among WDM optical signals are relatively narrow.
According to a first aspect of the present invention, for achieving the object mentioned above, there is provided an optical signal repeater, which is used at an optical fiber transmission system providing with an optical fiber transmission line, an optical transmitter, and an optical receiver. The optical signal repeater provides an optical signal amplifying means for amplifying wavelength division multiplexed (WDM) optical signals transmitted from the optical transmitter via the optical fiber transmission line, a pumping light source which outputs pumping light using for amplifying the WDM optical signals, an optical signal multiplexer which transmits the pumping light to the optical signal amplifying means, an optical splitter which splits the amplified WDM optical signals into two parts, a wavelength selection type reflection means which reflects only an optical signal having a specified wavelength in one part of the amplified WDM optical signals split at the optical splitter, an automatic output level controller to which the optical signal having the specified wavelength is inputted and which controls the pumping light outputting from the pumping light source so that the power level of the optical signal having the specified wavelength reflected from the wavelength selection type reflection means becomes a designated level, and a reflected optical signal transmitting means which transmits the optical signal having the specified wavelength reflected at the wavelength selection type reflection means to the automatic output level controller.
According to a second aspect of the present invention, in the first aspect, the optical splitter provides the reflected optical signal transmitting means.
According to a third aspect of the present invention, in the first aspect, the wavelength selection type reflection means is an optical fiber grating filter.
According to a fourth aspect of the present invention, there is provided an optical signal output level control method, which provides the steps of; inputting WDM optical signals transmitted from an optical transmitter via an optical fiber transmission line to an optical signal amplifying means, inputting pumping light to the optical signal amplifying means, amplifying the inputted WDM optical signals, splitting the amplified WDM optical signals into two parts, outputting main part of the split amplified WDM optical signals to an optical receiver via said optical fiber transmission line, inputting one part of the split amplified WDM optical signals to a wavelength selection type reflection means, reflecting only an optical signal having a specified wavelength in one part of the split amplified WDM optical signals at the wavelength selection type reflection means, inputting the optical signal having the specified wavelength to an automatic output level controller, and controlling the pumping light so that the power level of the optical signal having the specified wavelength becomes a designated level at the automatic output level controller.